1. Field of the Invention
The present invention relates to a dual-mode bandpass filter for use in, for example, communication apparatuses for microwave to milliwave bands.
2. Description of the Related Art Conventionally, various dual-mode bandpass filters have been proposed as bandpass filters for use in high frequency ranges (MINIATURE DUAL MODE MICROSTRIP FILTERS, J. A. Curtis and S. J. Fiedziuszko, 1991 IEEE MTT-S Digest).
FIGS. 22 and 23 are schematic plane figures illustrating conventional dual-mode bandpass filters.
In the bandpass filter 200 shown in FIG. 22, a circular conductive film 201 is disposed on a dielectric substrate (not shown). An input/output coupling circuit 202 and an input/output coupling circuit 203 are coupled to the conductive film 201 so as to be arranged at 90 degrees with respect to each other. An end-open stub 204 is provided at a position having a central angle of 45 degrees with respect to a portion having the input/output coupling circuit 203. This arrangement couples two resonant modes having different resonant frequencies, so that the bandpass filter 200 can operate as a dual-mode bandpass filter.
In the dual-mode bandpass filter 210 shown in FIG. 23, a substantially square conductive film 211 is disposed on a dielectric substrate. Input/output circuits 212 and 213 are coupled to the conductive film 211 to define an angle of 90 degrees with respect to each other. Also, a corner portion at a 135-degree position with respect to the input/output coupling circuit 213 is cut out. By providing a cutout portion 211a, the resonant frequencies of two resonant modes are made different, so that the coupling of the resonance in the two modes allows the bandpass filter 210 to operate as a dual-mode bandpass filter.
In addition, instead of the circular conductive film, a dual mode filter using a ring conductive film has been proposed (Japanese Unexamined Patent Application Publication Nos. 9-139612, 9-162610). In other words, a dual mode filter is disclosed in which a ring transmission path is used, input/output coupling circuits are arranged so as to define a central angle of 90 degrees, and an end-open stub is provided on part of the ring transmission path.
According to the conventional dual-mode bandpass filters shown in FIGS. 22 and 23, by forming one conductive film pattern, a two-stage bandpass filter can be formed, which can accordingly achieve an overall size reduction of the bandpass filter.
Nevertheless, the circular and square conductive film patterns have defects in that a broad pass band cannot be obtained because the patterns have a structure in which input/output coupling circuits are coupled with the above-described specific angle defined therebetween and the degree of coupling cannot be increased.
The shape of each bandpass filter is limited such that the conductive film 201 in the bandpass filter shown in FIG. 22 is circular and the conductive film 211 in the bandpass filter shown in FIG. 23 is substantially square. Accordingly, there is also a problem in that a degree of freedom in design is very low.
In the above-described bandpass filters, the dimensions and other characteristics of the conductive film determine the frequency band, so that it is difficult to adjust the band.
In order to overcome the problems described above, preferred embodiments of the present invention provide a dual-mode bandpass filter in which the above-described defects in the related art are eliminated, significant size reduction is achieved, and broadening of the band is achieved, while providing a high degree of design freedom.
According to a preferred embodiment of the present invention, a dual-mode bandpass filter includes a dielectric substrate having a pair of main surfaces, a metal film disposed on one of the main surfaces of the dielectric substrate or at a level within the dielectric substrate, a ground electrode disposed in the dielectric substrate or on one of the main surfaces of the dielectric substrate so as to oppose the metal film, with at least a portion of the dielectric substrate provided therebetween, first and second input/output coupling circuits coupled to the metal film, and at least one capacitor loaded to the metal film so that when an input signal is applied from one of the first and second input/output coupling circuits, two resonant modes generated in the metal film are coupled.
Preferably, the capacitor is provided in a portion of the metal film in which a resonant electric field that is relatively stronger than that of the remaining portion is generated.
The capacitor may include a capacitance lead-out electrode which is connected to the ground electrode and which is disposed in the dielectric substrate, and the layer of the dielectric substrate and the capacitance lead-out electrode and the metal film may have a capacitance therebetween.
The capacitance lead-out electrode may include a via hole electrode.
The capacitance lead-out electrode may further include a counter-electrode film that is disposed at an end of the via hole electrode and that is disposed in the dielectric substrate so as to oppose the metal film.
The plane shape of the metal film may be substantially rectangular, substantially rhombic, or substantially polygonal, or other suitable shape.
According to a dual-mode bandpass filter of preferred embodiments of the present invention, first and second input/output coupling circuits are coupled to a metal film that is partially formed on one of the main surfaces of a dielectric substrate or in the dielectric substrate. When an input voltage is applied from the first or second input/output coupling circuits, two resonant modes are generated in the metal film. Since at least one capacitor is loaded to the metal film so that the two resonant modes are coupled, a dual-mode-bandpass-filter operation is performed. In contrast to a conventional dual-mode bandpass filter in which points where the input/output coupling circuits are coupled must be disposed with respect to the metal film, which has a particular plane shape of circle or square, so as to define a central angle of 90 degrees, in the dual-mode bandpass filter of various preferred embodiments of the present invention, the presence, arrangement and function of the capacitance achieves the coupling of two resonant modes. Thus, the points at which the input/output coupling circuits are coupled do not always need to be arranged with respect to the metal film so as to define a central angle of 90 degrees.
In addition, by adjusting the capacitance and arranging the position of the capacitor, the bandwidth can easily be adjusted.
Accordingly, in preferred embodiments of the present invention, a bandpass filter can be provided in which a degree of design freedom is very high and a desired bandwidth can easily be obtained.
When an area in which the capacitor is provided is a portion of the metal film in which a resonant electric field that is relatively stronger than that of the other portion is generated, the two resonant modes are coupled such that in either resonant mode, a resonant electric field in the metal film portion in which the strong resonant field is generated is weakened by the provision of the capacitor.
In the case of the structure in which the capacitor includes a capacitance lead-out electrode being connected to a ground electrode and being disposed in the dielectric substrate and in which capacitance is led from the layer of the dielectric substrate between the capacitance lead-out electrode and the metal film, by adjusting the area of the capacitance lead-out electrode, the bandwidth can easily be adjusted. Also, a capacitor can easily be disposed in the dielectric substrate by using layered-ceramic-electronic-component production technology, which can further contribute to size reduction of the dual-mode bandpass filter.
In a case in which the capacitance lead-out electrode is a via hole electrode, the capacitance lead-out electrode can easily be formed by using a multi-layered ceramic substrate production method.
In a case in which the capacitance lead-out electrode includes a via hole electrode, and a counter-electrode film provided in the dielectric substrate so as to oppose the metal film, with a layer of the dielectric substrate provided therebetween, by adjusting the area of the counter-electrode film, the capacitance of the provided capacitor can easily be adjusted by a large amount.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention with reference to the attached drawings.